Requirements

In order to ensure the usability of JIT features (i.e.
create_jit_compiler), compile LLVM with –enable-jit as well.

About version numbers

The first two digits of ruby-llvm's version number refer to the
required major and minor version of LLVM. The third digit refers to the
ruby-llvm release itself. Because LLVM's api changes often, this
coupling between LLVM and ruby-llvm versions is useful.